The present invention relates to a catalytic converter assembly and in particular to a deNOx Catalytic Converter for reducing the nitrogen oxide (NOx) emissions from an internal combustion engine. More particular still, the invention relates to a selective catalytic reduction (SCR) system.
The exhaust emissions of internal combustion engines, and in particular diesel engines, tend to have high NOx content. A conventional method of combating this problem is to remove NOx from the exhaust gas using a catalytic converter located in the exhaust line. A particularly effective and proven system is to reduce the NOx to N2 and water in a suitable catalytic converter in the presence of ammonia. This is known as selective catalytic reduction (SCR).
In a typical SCR system the ammonia is produced by spraying an aqueous solution of urea into the exhaust stream upstream of the catalytic converter. At the high exhaust temperatures the water in the solution rapidly evaporates leaving solid urea which decomposes into ammonia when it reaches a temperature of around 150xc2x0 C. In order to ensure that there is the necessary reaction time to produce the ammonia from the urea, the urea solution is typically sprayed into the exhaust line between about 0.5 meters and 1 meter upstream of the catalytic converter. This is important to ensure that the urea both decomposes into ammonia and that the ammonia is adequately mixed in the exhaust gas steam.
The need to have a significant length of exhaust line between the point at which the urea is sprayed into the exhaust gas and the location of the catalyst limits the extent to which the size of the exhaust system can be minimised. There is however demand to have a compact exhaust system requiring as little space as possible for installation and operation. For instance, U.S. Pat. No. 5,832,720 proposes reducing the space requirement of an exhaust system by combining the catalytic converter with a noise reducing muffler by effectively utilising catalyst blocks as muffler elements. This does not however address the problem of the required separation of the urea spray and the catalyst.
U.S. Pat. No. 5,643,536 proposes a urea/ammonia metering system involving the provision of a hydrolysis catalytic converter to hydrolyse the urea into ammonia and water rather than relying on decomposition of the urea in the exhaust gas stream. Urea is introduced into the exhaust gas by spraying the urea into a chamber upstream of the hydrolyser in which it mixes with exhaust gas (perforated plates are provided within this chamber to prevent the urea solution from depositing on the walls of the chamber). This eliminates the need to have a substantial exhaust gas flow between the point of introduction of the urea and the catalyst blocks at the expense of a more complex and thus more expensive system.
The problems of the space requirement of conventional SCR exhaust systems are even greater in systems including a particle filter (typically incorporated in the exhaust system of a diesel engine). Conventional particle filters have catalyst coatings that destroy ammonia and thus must be located in the exhaust line upstream of the urea spray. This necessitates having two separate xe2x80x9cboxesxe2x80x9d in the exhaust system, one containing the particle filter and the other housing the SCR catalyst, the two being separated by a length of pipe into which the urea is sprayed.
It is an object of the present invention to obviate or mitigate the above disadvantages.
According to the present invention there is provided a catalytic converter assembly comprising:
a housing having an exhaust gas inlet and an exhaust gas outlet, an exhaust gas flow path being defined through the housing from said inlet to said outlet;
a first chamber enclosed by the housing receiving incoming exhaust gas;
a second chamber enclosed by the housing downstream of said first chamber and containing a catalyst bed;
an exhaust gas passage from said first chamber to said second chamber;
a perforated enclosure surrounding said passage within said first chamber to induce a region of turbulence in the exhaust gas flow within said enclosure immediately upstream of said passage;
an injector extending into said enclosure for injecting a reducing agent into said region of turbulence;
whereby the reducing agent is rapidly dispersed and mixed within the exhaust gas flow before reaching the catalyst bed.
Injection of the reducing agent into a region of turbulence in accordance with the present invention has been found to accelerate the dispersal and mixing process to a significant extent. For instance, in an SCR system in which a aqueous solution of urea is injected into the exhaust, the decomposition of the urea to produce ammonia is accellerated to such an extent that it is not necessary to have any significant length of exhaust gas flow between the point of injection of the urea solution and the catalyst. This enables the construction of an extremely simple and compact catalytic converter assembly. For instance, the first and second chambers maybe immediately adjacent one another separated only by a thin partition wall within the housing. Such an arrangement is not possible with the prior art SCR systems mentioned above.
It will be appreciated that the catalytic converter housing may define further chambers in addition to the xe2x80x9cfirstxe2x80x9d and xe2x80x9csecondxe2x80x9d chambers identified above. For instance sound attenuating muffler chambers may be provided between the inlet and the chamber referred to as the xe2x80x9cfirstxe2x80x9d chamber.
The perforated enclosure may take a variety of forms. In preferred embodiments the enclosure is defined by a tube which extends from the first chamber and which is perforated by the provision of holes in that portion of the tube which lies within the first chamber.
Embodiments of the invention may include a particle filter located within the first chamber upstream of the perforated enclosure. Thus, the invention can provide a compact catalytic converter which includes a filter thus avoiding the need to have a separate filter box upstream of the converter.
Further objects, advantages and preferred features of the present invention will be apparent from the following description.